1. Field of the Invention
This invention relates to digital communication systems and, more particularly, to determining the cable length and cable quality of the transmission medium.
2. Background
The dramatic increase in desktop computing power driven by intranet-based operations and the increased demand for time-sensitive delivery between users has spurred development of high speed Ethernet local area networks (LANs). 100BASE-TX Ethernet (see IEEE Std. 802.3u-1995 CSMA/CD Access Method, Type 100 Base-T) using existing category 5 (CAT-5) copper wire, and the newly developing 1000BASE-T Ethernet (see IEEE Draft P802.3ab/D4.0 Physical Layer Specification for 1000 Mb/s Operation on Four Pairs of Category 5 or Better Twisted Pair Cable (1000 Base-T)) for Gigabit/s transfer of data over category 5 data grade copper wiring, require new techniques in high speed symbol processing. On category 5 cabling, gigabit per second transfer can be accomplished utilizing four twisted pairs and a 125 megasymbol/s transfer rate on each pair where each symbol represents two bits.
Physically, data is transferred using a set of voltage pulses where each voltage represents one or more bits of data. Each voltage in the set is referred to as a symbol and the whole set of voltages is referred to as a symbol alphabet.
One system of transferring data at high rates is Non Return to Zero (NRZ) signaling. In NRZ, the symbol alphabet {A} is {xe2x88x921, +1}. A logical xe2x80x9c1xe2x80x9d is transmitted as a positive voltage while a logical xe2x80x9c0xe2x80x9d is transmitted as a negative voltage. At 125 M symbols/s, the pulse width of each symbol (the positive or negative voltage) is 8 ns.
An alternative modulation method for high speed symbol transfer is MLT 3 and involves a three level system. (See American National Standard Information system, Fibre Distributed Data Interface (FDDI)xe2x80x94Part: Token Ring Twisted Pair Physical Layer Medium Dependent (TP-PMD), ANSI X3.263:199X). The symbol alphabet for MLT 3 is {A}={xe2x88x921, 0, +1}. In MLT 3 transmission, a logical 1 is transmitted by either a xe2x88x921 or a +1 while a logic 0 is transmitted as a 0. A transmission of two consecutive logic xe2x80x9c1xe2x80x9d s does not require the system to pass through zero in the transition. A transmission of the logical sequence (xe2x80x9c1xe2x80x9d, xe2x80x9c0xe2x80x9d, xe2x80x9c1xe2x80x9d) would result in transmission of the symbols (+1, 0, xe2x88x921) or (xe2x88x921, 0, +1), depending on the symbols transmitted prior to this sequence. If the symbol transmitted immediately prior to the sequence was a +1, then the symbols (+1, 0, xe2x88x921) are transmitted. If the symbol transmitted before this sequence was a xe2x88x921, then the symbols (xe2x88x921, 0, +1) are transmitted. If the symbol transmitted immediately before this sequence was a 0, then the first symbol of the sequence transmitted will be a +1 if the previous logical xe2x80x9c1xe2x80x9d was transmitted as a xe2x88x921 and will be a xe2x88x921 if the previous logical xe2x80x9c1xe2x80x9d was transmitted as a +1.
The detection system in the MLT 3 standard, however, needs to distinguish between 3 levels, instead of two levels as in a more typical two level system. The signal to noise ratio required to achieve a particular bit error rate is higher for MLT 3 signaling than for two level systems. The advantage of the MLT 3 system, however, is that the energy spectrum of the emitted radiation from the MLT 3 system is concentrated at lower frequencies and therefore more easily meets FCC radiation emission standards for transmission over twisted pair cables. Other communication systems may use a symbol alphabet having more than two voltage levels in the physical layer in order to transmit multiple bits of data using each individual symbol. In Gigabit Ethernet over twisted pair CAT-5 cabling, for example, 5-level pulse amplitude modulated (PAM) data can be transmitted at a baud rate of 125 Mbaud.
In most ethernet receiver systems, cable quality or cable length is not monitored. Remote cable quality monitoring is a useful tool for system administrators. Difficulties with the transmission medium in a high-speed Ethernet application can lead to breakdowns in the receiver as a greater amount of distortion of the incoming signal due to the quality of the transmission channel becomes apparent. Therefore, there is a need for a receiver system that also indicates the length of the channel and the quality of the channel.
In accordance with the invention, a receiver that monitors the cable quality and indicates an estimate of the cable length is presented. The receiver monitors at least one parameter in the receiver and based on a known correlation between that parameter and the cable length, determines the cable length.
In one embodiment, the receiver includes a variable gain amplifier,.a gain control circuit, and a cable processor. The cable processor receives the gain g from the gain control circuit and, based on a correlation between the gain g and cable length that is stored in the cable processor, determines the cable length.
In another embodiment, the receiver includes an equalizer that has adaptively chosen multiplier coefficients and a cable processor. The cable processor receives the multiplier coefficients from a coefficient update circuit that adaptively chooses the multiplier coefficients and computes a metric based on the multiplier coefficients. The metric is chosen to be a monotonic function of the cable length and, by comparing the metric with a known correlation between the metric and cable length, determines the cable length.
In yet another embodiment, a cable processor compares a cable length as determined with the gain g from a gain control circuit with a cable length as determined from a metric of the adaptively chosen equalizer coefficients. If the two calculations yield substantially the same results, then the cable has a xe2x80x9cgoodxe2x80x9d quality. A xe2x80x9cpoorxe2x80x9d quality cable is detected when the two calculations yield substantially different results.
Embodiments of the invention are further explained below along with the following figures.